Poole is about 2.5h from London with 2 trains per hour for most of the day. There are regular, quick and convenient trains on the South West Trains service, but rail fares are generally expensive, so be careful.
There are coaches between Poole bus station and London about once an hour, via [wiki=b436d55f36cfbe8a085c8b75fb7fe98a]Bournemouth[/wiki].
From the east, take the M27 until it becomes the A31. Continue past Ringwood, and follow the signs for Poole. From the west, come in on the A35.
There are regular buses on both the Wilts and Dorset and Yellow Buses services, however Wilts and Dorset buses tend to go further, with links to [wiki=5a884401673693b0bdf379fefb7ec2b2]Southampton[/wiki], [wiki=fec3bc4c062141bc41f57ef54c27f6a0]Dorchester[/wiki] and [wiki=d8eae96cb899db873a46638a2f3bd448]Salisbury[/wiki].